Quantal Factors in the Joule-Thomson cooling of Hydrogen and Deuterium

Abstract

This paper presents a theoretical analysis of the Joule-Thomson cooling of normal hydrogen and deuterium from an initial temperature of 64 °K. Below 70 atm the cooling is greater for H₂ than for D₂, but at higher pressures it is less. It is found that the difference at low pressures is due to the lower heat capacity of H₂, associated with the more quantized rotation of its molecules. The difference at high pressures is due to the less negative enthalpy of imperfection of H₂, and this in turn can be attributed to the higher zero-point energy for translational motion in the lighter isotope. Both differences can be accounted for quantitatively.